The clinical heterogeneity in cystic fibrosis (CF) is only partly explained by mutations in the CFTR gene. Most CF patients have evidence of liver dysfunction and focal biliary cirrhosis (fibrosis), and a subset of these patients (5-7%) progresses to severe liver disease (CFLD), as defined by portal hypertension and multilobular cirrhosis. The development of CFLD has no relationship to specific CFTR mutations or other biomarkers, and there is currently no way to identify which CF infants will develop severe liver disease. The central hypothesis of this proposal is that the development of CFLD reflects the influence of non-CFTR "modifier" alleles (genes). This project is designed to identify associations between non-CFTR genes and CFLD, and test the biological effect of selected alleles on hepatic fibrosis in transgenic murine models. We hypothesize that polymorphisms in multiple genes, each with a conceptual or mechanistic link to liver disease, increase the risk for developing end-stage CF liver disease, and that interactions among these risk factors will define the pathophysiology of this disorder. To achieve our goals, we will study 400 CF patients with well-documented severe liver disease and portal hypertension, and 400 gender and genotype-matched CF patients > age 15 years who have no evidence of CFLD. We propose to identify heritable risk factors for the development of CFLD by evaluation of functional sequence variants within, and single nucleotide polymorphisms associated with, multiple genes associated with CFLD pathogenesis. To test ("validate") the biological effects (impact) of selected genetic alleles on liver fibrosis, we will develop transgenic mice homozygous for deltaF508, who are also expressing an additional candidate gene modifier allele. Better definition of the complex genotypes that increase risk for severe liver disease in CF will allow early identification of CF infants predisposed to develop end-stage liver disease, and thereby allow testing of currently available therapies. Better understanding of the pathobiology of hepatic fibrosis in CF will identify novel targets to prevent (or reduce) the development of CFLD.